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Perennial pepperweed belongs to the mustard or Brassicaceae family. It is an erect, branching perennial weed that grows one to three feet high, but may reach heights of eight feet in wet areas.  The base of the stems is semi-woody.  The roots enlarge at the soil surface to form a woody crown.

Dense white flower clusters of six to eight tiny blossoms occur near the end of stems around mid-June. It is a prolific seed producer, capable of producing more than six billion seeds per acre of infestation. Nearly microscopic, reddish-brown seeds occur in an elongated pod (less than one eighth inch in diameter) and are rounded, flattened and slightly hairy.

In addition to seeds, it spreads by creeping underground roots (rhizomes) which may grow to a length of ten feet. New plants shoot up from the underground root and enable it to form dense monocultures that block sunlight from reaching the soil, thus suppressing the growth of other plants.

Spread: The plant commonly travels in rivers and irrigation systems as seeds and rhizomes from eroded banks. Flood irrigation carries plant propagules into hay meadows, pastures and other irrigated lands.   Perennial pepperweed is also carried in contaminated topsoil used as fill in construction and landscaping sites.  Seeds are transported when they attach themselves to machinery and vehicle tires.  Livestock, waterfowl and dried flower arrangements disperse seeds long distances.

Threat: Perennial pepperweed is a very competitive species that crowds out desirable vegetation and results in dense monocultures and a decrease in biodiversity. When established along rivers and streams, the plant interferes with the regeneration of willows and cottonwoods, reducing cover and food availability for birds.  The accumulation of semi-woody stems negatively impacts nesting habitat for wildlife.

Perennial pepperweed poses a large threat to hay meadows. It is introduced through irrigation ditches and, once established, can decrease protein content and digestibility of hay.   In areas that are not mowed annually, semi-woody stems can accumulate and hinder grazing.  Although there is no scientific evidence, it is believed that pepperweed is toxic and could pose a threat to livestock.

Article source: “Perennial Pepperweed”,  Mangold, J and Sheley, R., MSU Extension MT199906AG Revised 5/12

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